Strong Optical Second Harmonic Generation from Suspended Graphene Sheets
Kung-Hsuan Lin (林宮玄)1*, Shao-Wei Weng (翁紹尉)1, Po-Wei Lyu (呂柏暐)1,2, Tsong-Ru Tsai (蔡宗儒)2, Wei-Bin Su (蘇維彬)1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
2Institute of Optoelectronics Sciences, National Taiwan Ocean University, Keelung, Taiwan
* presenting author:林宮玄,
Graphene, a monolayer of carbon atoms arranged in honeycomb lattice, has become a subject of intense interest due to its outstanding thermal, mechanical, electronic and optical properties. Despite the graphene’s thickness is only one atomic layer, previous studies revealed that its nonlinear optical response is strong. Most of previous studies of optical nonlinearity in graphene are related to third-order nonlinearity because second-order susceptibility of an ideal graphene should be zero due to its centrosymmetric structure [1]. Direct current (DC) on graphene can break the symmetry, and strong second harmonic generation (SHG) has been studied recently [2]. But, DC-induced SHG is still basically a third-order nonlinear process that includes one DC electric field and two photons. At the surface of a centrosymmetric lattice system or at the interface of two materials, SHG is permitted due to the broken symmetry. Surface SHG has been observed from single-layer and few-layer graphene on SiO2/silicon substrates [3]. However, SHG is not expected from a freestanding graphene in air due to the 3D centrosymmetric system.

In this presentation, we report the observation of SHG from suspended graphene sheets [4]. Although SHG from an ideal freestanding single-layer graphene sheet is forbidden, the measured strong SHG signal from the suspended graphene arises from the curvature of the sheet in the long range. Since the ripples of graphene sheets are expected to strongly influence electronic properties, there are many interests of bandgap engineering by controlling the local strain, i.e., local curvature. SHG mapping techniques could have great potential to rapidly identify the nanometer-height ripples in micron lateral scales because the SHG from suspended graphene sheets primarily originates from the long range curvature.

[1]M. M. Glazov, Jetp Lett. 93, 366 (2011).
[2]Yong Q. An, Florence Nelson, Ji Ung Lee, and Alain C. Diebold, Nano Lett. 13, 2104 (2013).
[3]Jesse J. Dean and Henry M. van Driel, Appl. Phys. Lett. 95, 261910 (2009).
[4]K.-H. Lin, S.-W. Weng, P.-W. Lyu, T.-R. Tsai, and W.-B. Su, Appl. Phys. Lett. 105, 151605 (2014).

Keywords: graphene, 2D material, second harmonic generation, nonlinear optics